This invention concerns a method to obtain transverse vibrations in the walls of a crystaliser in an ingot mould by means of a pulsation in the cooling fluid.
The invention is applied in the field of continuous casting of billets, blooms or slabs of any type or section, in order to reduce friction between the cast product and the walls of the crystalliser, thus allowing an increase in the speed of casting and reducing the risk of breakout in the skin of the product as it is formed.
The problems connected with trying to reduce the force needed to extract the cast product from inside the crystalliser are well known to the state of the art.
It is known in fact that, as the skin solidifies, it tends to stick to the walls, at least in the upper part of the crystalliser, generating considerable friction in the extraction stage.
To facilitate the detachment of the skin from the wall, the state of the art covers the method of generating longitudinal, mechanical oscillations on the ingot mould which facilitate the extraction of the cast product and allow the casting speed to be increased and the surface quality of the product leaving the crystalliser to be improved.
It is also known that in the lower part of the crystalliser, the skin which has by now solidified tends to become detached from the walls, thus creating an air gap which causes a reduction in the heat exchange between the cooled wall and the solidified skin, and therefore a reduction in the heat flow removed from the molten metal through the wall of the crystalliser.
The present applicant, in the European patent application EP-A-0686445, described the use of a crystalliser with thin walls associated with a method to control the deformations of said walls; that invention uses the regulation of the pressure of the cooling fluid which runs in the transit channel adjacent to the walls so as to compensate for the different shrinkage of the skin of the cast product through the crystalliser according to the type of steel and the casting speed.
According to that document, the walls of the crystalliser assume an elastic state in relation to the different pressures of the cooling fluid which runs outside them, in such a way as to cancel, in the first portion of the crystalliser, the negative taper induced by the heat field and, in the lower part of the crystalliser, in such a way as to minimise the air gap which is created between the solidified skin and the walls.
These pressures are calculated to obtain the desired deformation of the walls and are maintained substantially constant until the casting parameters, particularly the type of steel and the casting speed, are changed.
The variable pressure of the cooling liquid is therefore used to deform the sidewalls, not to make them vibrate elastically.
DE-A-19.547.779 discloses a device which uses the pulsation of the pressure of the cooling liquid delivered to a support device, (14, 19, 20) which has a yielding effect to obtain vertical, or in any case, longitudinal oscillations, in a crystalliser.
The support device is identified as a bellows device (FIG. 1), a fluid cylinder (FIG. 2) or a pair of disk springs (FIG. 3).
The pulsation in the pressure of the cooling liquid causes a vertical oscillation of the support device (14, 19, 20) which causes a corresponding vertical oscillation of the crystalliser (2) associated with the support device (14, 19, 20).
This device replaces, or cooperates with, the conventional mechanical means of vertical oscillation of the crystalliser so as to assist the descent of the molten metal and detach the solidifying skin from the inner sidewalls of the crystalliser.
In the device as disclosed by DE'779, the pulsation in the pressure of the cooling liquid does not act directly on the sidewalls of the crystalliser in order to cause an elastic vibration thereof in a tranverse direction to that of the casting, it acts by means of the support devices (14, 19, 20) with a yielding effect so as to induce vertical oscillations on the crystalliser (2).